The present invention relates to a fuel rod for a nuclear reactor which contains uranium dioxide (UO.sub.2) or a mixed-oxide (MOX) of uranium dioxide and plutonium dioxide as a nuclear fuel.
In a thermal neutron reactor in current use, a nuclear fuel containing gadolinium (Gd) which is a combustible reactivity control material is used in order to control the excess reactivity and to reduce the local peaking factor to thereby improve the burnup of the nuclear fuel.
Gd-containing nuclear fuels used heretofore include a homogeneous pellet consisting of a solid solution of gadolinium oxide (Gd.sub.2 O.sub.3) in UO.sub.2 or MOX and a double pellet consisting of a solid solution of Gd.sub.2 O.sub.3 in only the central portion of the UO.sub.2 or MOX pellet.
These pelletized fuels are used by piling up a number of the pellets to form a pellet stack, sealing the stack on a cladding tube to form a fuel rod, and bundling up a plurality of the fuel rods to form a fuel assembly, which is placed in a reactor core.
However, the above-described homogeneous pellets have a defect in that the power control effect of Gd is so remarkable that the fuel rods having a very low power are unevenly distributed in the assembly. Thus there are nuclear and thermal restrictions in the homogeneous pellets.
In addition, since the formation of the solid solution of Gd.sub.2 O.sub.3 in UO.sub.2 or MOX is limited, it is impossible to select the optimum Gd.sub.2 O.sub.3 concentration depending on the performance of the nuclear fuel.
Further, in the manufacture of the homogeneous or double pellets, a manufacturing line for Gd-free pellets and one for Gd-containing pellets are necessitated in a nuclear fuel manufacturing facility and the contamination of the Gd-free pellets with Gd must be avoided. Therefore, the manufacturing line for the Gd-containing pellets by using powdery Gd.sub.2 O.sub.3 must be completely separated from the ordinary manufacturing line for the Gd-free pellets and two or more inspection facilities are required, which remarkably increases the manufacturing cost of the nuclear fuel.